In servo systems and particularly in position loop servo systems used for positioning a movable member by use of a position feedback signal, there exists the problem of mechanical resonance. Mechanical resonance is oscillation of the circuit and movable member at a frequency or frequencies at or near the natural resonant frequency of the system. Such occurs because of the play or tolerance within the mechanical system and the inability of the servo system to distinguish between actual position error and error due to the looseness in the mechanical system. Attempts to eliminate or reduce the possibility of such resonance have included the use of filters to attenuate the position error signals at frequencies near the natural resonant frequency of the system.
A first problem arises from attenuating the feedback signal with filters in that the overall reaction of the system is reduced thereby requiring a longer time period to position the movable member at a desired location, especially in the areas of the attenuated signal frequencies. Still another problem results from the fact that such attenuation is usually accomplished by an electrical filter. However, such filters also usually cause a phase shift of the feedback signal and at times the phase shift in the feedback signal can cause the system to perform worse than it did without the filter even though some attenuation of the feedback signal is achieved. Therefore there remains the problem of compensating for the magnitude and the phase shift and one circuit for accomplishing this is shown in U.S. Pat. No. 3,808,486 entitled: Selective Frequency Compensation For A Servo System, issued on Apr. 3, 1974. In this patent the gain is increased in a particular frequency range to reduce the possiblity of instability caused by mechanical resonance in the system at other frequency ranges. However there results a phase shift and to compensate for this shift this patent describes a special circuit to be utilized in such feedback systems. There also remains the need to dither or cause a slow oscillation in a position loop servo system to reduce the effects of friction and inertia of the movable member. However, most efforts to damp a position loop servo system will exclude the possibility of dithering. This exclusion of the dithering operation slows the functioning of the system or produces a large steady state error.
It is therefore the object of this invention to provide a means for preventing high frequency mechanical resonance instabilities in a position loop servo system, while not substantially affecting the operation of the system in other lower frequency ranges. It is a further object of this invention to enhance the operation of a position loop servo system by providing means for attenuating the position error signal at higher frequency ranges and enhancing the position error response of the system at other frequency ranges.